Alzheimer's disease is characterized by the presence of numerous amyloid plaques and neurofibrillatory tangles present in the brain, particularly in those regions of the brain involved in memory and cognition. β-amyloid peptide (Aβ) is a 39-43 amino acid peptide that is major component of amyloid plaques and is produced by cleavage of a large protein known as the amyloid precursor protein (APP) at a specific site(s) within the N-terminal region of the protein. Normal processing of APP involves cleavage of the protein at point 16-17 amino acids C-terminal to the N-terminus of the β-AP region, releasing a secreted ectodomain, α-sAPP, thus precluding production of β-AP. Cleavage by β-secretase enzyme of APP between Met671 and Asp672 and subsequent processing at the C-terminal end of APP produces Aβ peptide, which is highly implicated in the etiology of Alzheimer's pathology (Seubert, et al., in Pharmacological Treatment of Alzheimer's disease, Wiley-Liss, Inc., pp. 345-366, 1997; Zhao, J., et al. J. Biol. Chem. 271: 31407-31411, 1996).
It is not clear whether β-secretase enzyme levels and/or activity is inherently higher than normal in Alzheimer's patients; however, it is clear that its cleavage product, Aβ peptide, is abnormally concentrated in amyloid plaques present in their brains. Therefore, it would be desirable to isolate, purify and characterize the enzyme responsible for the pathogenic cleavage of APP in order to help answer this and other questions surrounding the etiology of the disease. In particular, it is also desirable to utilize the isolated enzyme, or active fragments thereof, in methods for screening candidate drugs for ability to inhibit the activity of β-secretase. Drugs exhibiting inhibitory effects on β-secretase activity are expected to be useful therapeutics in the treatment of Alzheimer's disease and other amyloidogenic disorders characterized by deposition of Aβ peptide containing fibrils.
U.S. Pat. No. 5,744,346 (Chrysler, et al.) describes the initial isolation and partial purification of β-secretase enzyme characterized by its size (apparent molecular weight in the range of 260 to 300 kilodaltons when measured by gel exclusion chromatography) and enzymatic activity (ability to cleave the 695-amino acid isotype of β-amyloid precursor protein between amino acids 596 and 597). The present invention provides a significant improvement in the purity of β-secretase enzyme, by providing a purified β-secretase enzyme that is at least 200 fold purer than that previously described. Such a purified protein has utility in a number of applications, including crystallization for structure determination. The invention also provides methods for producing recombinant forms of β-secretase enzymes that have the same size and enzymatic profiles as the naturally occurring forms. It is a further discovery of the present invention that human β-secretase is a so-called “aspartyl” (or “aspartic”) protease.